“Mitigate” is the only verb that fits well within the context of the sentence (your third option). You will see in the definition that one meaning of mitigate is “to cause to become less harsh or hostile”. It seems to fit in that the serious attitude of the unsmiling Tom could be made less harsh by “my sense of humor”. Two synonyms for “mitigate”, listed after that definition, are “mollify” and “soothe”, and they seem to fit the sentence very well.
Using “reduce” is possible, but not it’s not idiomatic to talk about “reducing” attitudes. (”Lessen”, being like “reduce”, might have worked better and is getting closer in meaning to “mitigate”)
“Animate” seems to be the worst option. This verb carries a sense of increasing or enlivening something, making it almost an opposite of the required meaning.